text:
What does the expression "Normally with hull loss" mean?
This is a question from a non-english speaking person.
I encountered the expression "Normally with hull loss" in this safety categorization:

(image source: EASA CS-25)
Why is that not only "hull loss". Does the expression means it is not sure but probable?

You need to provide a larger context. It's possible this was to distinguish an accident from a catastrophic accident. A hull loss is indeed an accident, but it is catastrophic.

Welcome to Aviation.SE! I added an image from EASA CS-25, which is probably where you got the expression from. If not, please edit again and replace it with your source.

Thank you Bianfable. It is from this source indeed.

Interestingly, the death of a single crew member makes the incident catastrophic, but if a small number of passengers suffer lethal injuries, it is ranked only hazardous

@usr-local-ΕΨΗΕΛΩΝ I would read that table as an overall characterization of the different incident severities.

@usr-local-ΕΨΗΕΛΩΝ A single flight-crew loss could lead to losing the entire aircraft, whereas loosing several passengers isn't good, but has a significantly lower chance of causing serious to fatal injuries to the rest.

Yes, effects on flight crew, occupants and aeroplane have to be estimated before evaluating the severity classification of the situation

As I understand the phrase, it is saying, "Catastrophic Failure normally results in hull loss."
But not necessarily; It is possible to have a catastrophic failure without total hull loss.
"Normally" here is a synonym for "typically".
Hull Loss is typical for this type of failure.  But it is also possible (although more rare) to have a catastrophic incident without hull loss.

I haven't been able to find a good example of such an accident. This case, where the captain died in flight, might qualify (since fatalities under flight crew is listed as catastrophic), but his death was not really the result of the accident.

@Bianfable  UA Flight 811 Explosive Decompression would be a possible example. Nine passengers were lost, so it meets the criteria for Catastrophic. As to hull loss it would depend on how you classify it. In this case the a/c survived to a safe landing so it wasn't lost in the event. I can't find a reference to its disposition after the investigation, but I expect it was likely scrapped as uneconomical to repair.

@Gerry That's an excellent example: "The aircraft was repaired, re-registered as N4724U in 1989, and returned to service with United Airlines in 1990." (Wikipedia)

One might also say that in this context the "normally" means that hull loss is the most likely outcome of the incident/accident.

@Bianfable: I would expect that many pre-flight checklists occasionally trigger failures that would be classified as catastrophic, but don't end up causing any injury, fatilities, or hull loss because the planes are stationary on the ground when they occur (which is why the checks are done before taking off).

https://en.wikipedia.org/wiki/List_of_airline_flights_that_required_gliding The Gimli Glider springs to mind (all engines failed due to fuel exhaustion, glided to land on a racetrack and the hull had 20+ years more flight after repairs--don't try that one at home). A couple on that page are even crazier saves, but most of them do result in hull loss.

@user3067860 Yes, but why would the Gimli Glider be classified as catastrophic (nobody died). According to the table, I would classify it as hazardous because of "large reduction of functional capabilities or safety margins" for the aircraft.

@Bianfable https://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243 would be a good example of a catastrofic failure, 1 death, 65 injured, 8 serieus injured. All passengers were wearing seatbelts. The aircraft was damaged beyond repair

@Ferrybig "The aircraft was damaged beyond repair" -> that implies it was a hull loss. I was wondering about accidents classified as catastrophic that were not hull losses, like UA Flight 811 found by Gerry.

@Bianfable I found an answer in an FAA AC, I added it as a full answer, but the difference is a catastrophic failure (yes) vs catastrophic effects (not necessarily).

"Normally with hull loss" is just a way to say that "if this were to occur, that would be the normal outcome". The whole table is used to classify possible events happening during flight or on the ground based on the probability of it happening and the consequences.
While I couldn't find the same table in your link, I found a similar one other places, such as on page 5 here.

@ymb1 Yes, that's the correct interpretation, I think.  The footer of that table should probably have been formatted to be a header.

I found the answer to this on page 11 of FFA Advisory Circular 2023.1309-1E where they define "catastrophic" (this has the same chart as yours later on, so I think it should also apply to yours):

Notes:  (1)  The phrase “are expected to result” is not intended to require 100 percent certainty that the effects will always be catastrophic.  Conversely, just because the effects of a given failure, or combination of failures, could conceivably be catastrophic in extreme circumstances, it is not intended to imply that the failure condition will necessarily be considered catastrophic.

The next note is also helpful:

(2)  The term “catastrophic” was defined in previous versions of advisory materials as a failure condition that would prevent continued safe flight and landing.

By that reading, I think The Gimli Glider experienced catastrophic failure conditions without actually experiencing catastrophic effects, since the failure* would probably have caused catastrophic effects except that the flight got very, very, very improbably lucky.
* Total engine loss due to fuel exhaustion, in a plane that really wasn't designed for total engine loss.

Nice find! Makes sense to distinguish between the failure condition and the actual effect it had...


text:
Can I fly RNAV 5 with only INS?
First of all, I'm not a native English speaker, and this is my first question, so please understand that it can be very awkward.
My military C-130H aircraft has DME/VOR, TACAN, ILS, and INS (EGI, LN-100R). (These are navigation equipment from the 1980s.). There are no GPS, no FMS.
My question is, can this aircraft use Y Airway?
Since I usually fly VFR in local area, I have little knowledge about RNAV.
As far as I understand, RNAV 5 is an En Route phase, and requires equipment that can use ground stations(like VOR) and INS or GNSS equipment.
Since my aircraft has VOR/DME and INS, is it capable of RNAV 5? If so, can I use Y Airway?
If not, is a separate certification or authentication required to use RNAV?

In order to fly RNAV-5, the aircraft equipment needs to be approved to do so. This Air Force publication indicates that C-130H aircraft are capable of RNAV-5 navigation, subject to some requirements on aligning the inertial navigation system.
Exactly what that will permit you to fly depends on where in the world you are. In the US, all airways are RNAV-2 or stricter, so RNAV-5 is useless. You say in comments that you are in Japan, and it does appear that Japan has some RNAV-5 airways.
It should go without saying that you should clarify this with someone with local knowledge. There may be local regulations or rules I am unaware of.

Thank you for answer. Then, among the RNAV routes such as Y, T, and Q, are there any routes that I can use?

@Tom Are you in the US? All RNAV airways in the NAS are RNAV 2 or better, so they are all off limits. Elsewhere in the world the answer may be different.

No I am in Japan. According to your answer, my aircraft may be capable of RNAV 5. So what does RNAV 5 enable? They said RNAV 5 is the enroute phase, so I thought it meant the RNAV airway.

@Tom Edited my answer. Some of the information in it before was US-specific.

Thank you very much

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
How to find the equations used in an N2 diagram?
I am currently pursuing my research internship whose aim is to validate the airframe design estimation by FastUAV, an optimization framework (written in OpenMDAO) specifically tailored to unmanned aircraft. I wonder how they have defined certain variables/parameters. Since the code has been written by someone else who has not left any accurate documentation, the only way to do that could be look at how they used those parameters in the computations. Hence, being new to UAV and N2 diagrams, my question is, how could I exploit this tool (N2 diagrams https://openmdao.org/newdocs/versions/latest/features/model_visualization/n2_basics/n2_basics.html) to look at the hidden equations?

Isn't the source code available? I think anyway that this question has more to do with coding than aviation being N2 diagrams related to the openmdao framework...

The OpenMDAO developers are active on StackExchange -- I don't immediately recall which forum they frequent, but you should be able to find them on here.
If you only have a printed N2 diagram, you won't be able to find out much other than the chosen breakdown into components (boxes) and the abstract inputs/outputs from one to another.  Without the code, you can't do what you want.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Why did LH412 fly in circles, far from its destination?
On 5 June 2022 Lufthansa flight 412 from Munich to Newark flew 3 clockwise circles over the sea just west of Brittany:

(FlightAware.com)
Why did it fly these circles? It was many hours away from the US and ended up diverting to St. John's (YYT).

Assuming you are referring to yesterday's (June 5 2022) LH-412 flight (Lufthansa) it ended up being diverted to YYT en-route. No reasons have been published, yet, that I can locate. Could be any number of reasons, from unruly passenger(s) who calmed down enough to continue, to some sort of non-critical hardware issue. Also, it wasn't "flying in circles far from its destination" as much as it was doing so, while still within divert distance to an airport in Europe. i.e. it hadn't yet gotten to a point-of-no-return over the ocean.

My first thought was holding prior to oceanic entry to increase separation with preceding aircraft, which is sometimes causing issues. However, I cannot see any other aircraft entering oceanic airspace on FR24's playback feature in front of them.

Sounds like an answer @CGCampbell

@FreeMan not at all, I don't yet know why the circular pattern was done, which was the question asked. At the time I posted my comment, we didn't yet know for sure what flight was actually being asked about.

Gotcha, @CGCampbell. All I could see was time stamps of "x hours ago" and lost the fine details...

There was a special needs passenger on board who was terrified and was having physical meltdowns. The crew tried to keep him calm but he became too agitated.

Do you have firsthand knowledge of this flight? If so you should say so; if not you should add supporting references.


text:
Who gives us transition level when it varies?
Who gives us transition level when it varies?
ATC, ATIS, any others?

Check the country's AIP, ENR section.
ATIS, ATC or FIS is a good bet otherwise but it remains a bet.
In some EASA countries, you can calculate it yourself if the Transition altitude is fixed. E.g. Germany's TA is 5000ft, with the transition layer a minimum of 1000 ft. So with a QNH of 1014 the TL will be FL60, 978-1013 it'll be FL70 and 977- will be FL80. Others are different but they don't hide the information, it's in the AIP.

What do you mean by ENR section? Ok great mate,  so you bet as in you are sure? So ATIS,ATC and FIS...Isn't transition altitude always fixed or known so to say...so you can always find transition level...but I know that some countires have a transition layer big enough like 2000 ft thay they fix the  transorion level....

I'm not a betting man, I like to be sure so I read the AIP of the country I fly over or through. Every AIP has a number of sections, e.g. AD for the Aerodrome info and ENR for enroute. Here's e.g. Belgium's Transition Altitude and Level:
https://ops.skeyes.be/html/belgocontrol_static/eaip/eAIP_Main/html/eAIP/EB-ENR-1.7-en-GB.html
Or here's how the UK works: https://nats-uk.ead-it.com/cms-nats/opencms/en/Publications/AIP/Current-AIRAC/html/eAIP/EG-ENR-1.7-en-GB.html

Great mate, very kind.Amazing✈✈❣❣

@YosefJabbour Glad it helped. Please accept an answer to mark this as answered.

Usually ATIS, or the atc which first clears you to an altitude if there is no ATIS. In some cases (e.g US) it is fixed so there is no need for it to be given. In those cases its usually charted in the arrival/approach chart.

What is the transition level in the us? Also why is it fixed? Isn't the value of transition level dependent on qnh = local pressure conditions...( some countries fix the tranaition level if the transition layer distance is large...)

So is the Transition Level in the US fixed to FL200? What happens if the QNG drops below 977? The vertical separation between aircraft cruising at 18.000ft and at FL200 would be only 970ft or even less with lower air pressure?

@pcfreakxx In the US, the lowest usable flight level is FL180, FL190 or FL200 depending on QNH to ensure 1000 ft separation with 17,000 ft (SKYbrary).

@Bianfable Thanks for your explanation! I was falsely assuming the TA was 18.000ft in the US. But having a TA of 17.000ft and variable TRL of FL180-FL200 makes perfectly sense. It’s the same here in Germany, although with different values (TA 5.000ft, TRL: FL60-80).

@pcfreakxx Technically TA is at 18,000 ft in the US. It's just not the highest usable altitude (unlike in Europe).

@Bianfable That doesn’t make sense to me. Does that mean you are using completely contrary definitions of TA and TRL than the rest of the world? Per ICAO definition the local QNH shall be used at or below TA and TRL is the lowest usable FL.

@pcfreakxx Did you read the SKYbrary article I linked? It explains the differences between North America and Europe quite nicely.

@Bianfable I just read it, thank you so much! I have to admit: this seems really counterintuitive, lucky I don’t have to remember.


text:
How to determine the base of stratus clouds?
For cumulus clouds this applies

Calculate the 'spread,' where you're taking the current surface temperature and subtracting the dew point.
Divide the spread by 4.4 (if temperatures are in °F) or 2.5 (if temperatures are in °C), then multiply by 1000. This will give you cloud base in feet above ground level.

But what about stratus clouds?

This is pretty much a purely meteorological question, and while weather does apply to aviation, (as I have argued before...) this one is off topic.  VTC.

Maybe if you added "Do pilots flying big airlines care about..." it would be on topic?  (but then the answer would be "no", and you wouldn't learn how to do your calculation.)

There is something wrong with that formula for cumulus' bases; I would say it's wrong by about a factor of 3 (it gives bases three times as high as reality).

There is no formula for determining the height of the base of stratus clouds based on the ground-level temperature and dewpoint (or relative humidity).  Unlike convective cumulus clouds, stratus clouds are not formed by parcels of air rising up from the ground.  (Actually, cumulus clouds are not always formed by air rising from ground level either, but they usually are.)

Just a note to emphasize the right word: stratus clouds are not formed by parcels of air rising up from the ground.  Stratus (and cumulus) are caused by rising air, it's just that the source parcel for both may or may not be near ground level.


text:
Source of Air in pressurised doors
In pressure cabins, pressurisation is very important. To keep the pressurisation level balanced, rubber seals are used.
Now what is the source of the air used to maintain this level of pressurisation?

Are you asking about the pressurisation of the cabin in general or specifically about how pressurised air can be used inside the rubber seals?

You will generally find two kinds of seals:

Retractable pressurized seals that run off pressurized bleed air, maybe tapped off the deicing system or the high pressure side of the air conditioning system, and extend out from grooves in the door frame when the handle is locked.  There will normally be a pressure reservoir tank that can sustain pressure to the door seal if the bleed source fails.
Passive seals that are like tubular gaskets around the door frame. There will be vent holes in the tube exposed to the cabin air side when the seal compressed with the door closed, that allows ambient cabin air to pressurize the seal relative to the outside.

Neither of these types of seals works perfectly and pressurized cabins normally leak a fair amount past the various seals and gaskets, and through rivet holes and skin joints, becoming leakier and leakier as the airplane ages.


text:
How can an ultralight be slowed down for landing in a tailwind?
When flying an ultra light and I have a tail wind, and, with the engine at idle how do I slow down to land ?

Why do you think it's a good idea to land downwind?

Turn the aircraft 180 degrees. ;)

You don't change anything expect perhaps use short field landing technique to minimize your landing groundspeed, but otherwise you take the high ground speed. In light winds this isn't that big a deal if the runway is long enough.
I regularly land with minor tailwind components, under 5 kts, when it's more convenient and there are no traffic conflicts and space isn't critical.  At a very large uncontrolled airport (10000 ft runway) I used to fly to regularly, where I had to share the runway with arriving large jets hauling freight from time to time, I would often use the reciprocal runway when following a cargo jet's arrival, taking the tailwind, in order to completely avoid the jet's wake, as long as the tailwind component was under about 12-15 knots.  When you have all that room, the main issue is tire wear from the high touchdown speed.
Glider pilots train for downwind landings all the time, since it's a normal result of a low altitude rope break.  The biggest problem isn't so much the high landing ground speed, it's avoiding overshooting your intended landing zone with the huge increase of effective glide angle with the wind behind you.  Glider students are often very shocked when they discover this.
Even in an ultralight with a fairly steep glide angle, if you did a turnback to land downwind after an engine failure from 400 ft, you might also find your biggest problem is not overshooting, but in any case you would just use the minimum normal airspeed (short field) to land.

Wait, does the effective glide angle increase or decrease with a tailwind?  I would think the glide ratio would increase, which would correspond to a decrease in effective glide angle.

Depends on the reference.  I was using from vertical, which would be an increase.

I think this anecdote bears directly on your question--
I often shoot downwind landings in winds up to 10 mph in a lightweight radio-controlled model airplane of tailwheel configuration.  The key is to fly by reference to airspeed, not groundspeed.  (One way to accomplish this in a radio-controlled model airplane, which lacks an airspeed indicator, is to learn where the elevator trim control needs to be set for a nice power-off glide, with hands off the controls, that is comfortably faster than stall speed and gives you a little extra airspeed "padding" to allow an authoritative round-out or "flare".)  Avoid any temptation to deviate below this selected target airspeed when landing with a tailwind, just because the groundspeed is high.
Downwind landings often actually tend to be smoother than upwind landings, because if the aircraft accidentally "balloons" upward during the landing flare, the wind gradient will be working to reduce the airspeed, not to increase the airspeed, reducing the height gained during the "ballooning".  Conversely, if the aircraft is dropping-- for example if the aircraft is rapidly settling toward the ground after excessive "ballooning"-- the wind gradient will be working to increase the airspeed, not to decrease the airspeed, so this tends to lead to a smoother touchdown.
In an aircraft of tailwheel configuration, however, it's often wise to limit your downwind landings to touch-and-goes (or touch-roll-and-goes, with an extended interval of high-speed taxi on the main wheels with the tail in the air) rather than attempting to slow to a full stop.  As the aircraft decelerates with the wheels rolling on ground, a point comes where the airspeed over the control surfaces drops to zero, and then reverses, making the plane extremely vulnerable to ground-looping.  Tricycle-gear aircraft are generally a much better choice than tailwheel aircraft, for full-stop landings with a tailwind!
A few more words of caution-- the overshooting tendency on final approach with a tailwind is not caused by the high groundspeed alone.  The wind gradient actually reduces the aircraft's sink rate.  While we've already noted that this can lead to a smoother touchdowns, it can also move that touchdown much further down the runway than the pilot intended, unless he or she adequately anticipates and allows for these effects.  And you should also keep in mind that if you attempt a go-around with a tailwind, not only will your high groundspeed reduce your climb angle or gradient, but the wind gradient will actually reduce your climb rate as well, especially in the first few tens of feet or meters above the ground.
If you really want to slow the aircraft on final approach for the slowest touchdown that is safely possible, you do it exactly the same way when landing with a tailwind as you would do when landing with a headwind (e.g. when landing on a rough field) -- apply flaps, carry some power so you can safely operate a bit on the "backside of the power curve" without incurring a tremendous sink rate, etc-- to the extent that these techniques are appropriate and applicable in your particular aircraft.  Whatever you do, don't let the high groundspeed influence you into choosing a slower-than-appropriate airspeed.
See also the answers to this highly-related ASE question: How to land a sports plane with tailwind? .  Note several references to the effects caused by the wind gradient.


text:
What type of visa do pilots who fly from one country to another need?
I am little curious about international travel. I would like to know what type of visa allows pilots and cabin crews to fly from one country to another? I mean suppose I am pilot of Japan airline and I have trip a for Netherlands. How could I enter to the Netherlands with Japanese citizenship?

Why is triplane one of the tags?

Wild guess: typo. Edited it away.

I suppose you are asking about a situation where the pilot leaves the aircraft and exits the airport into the foreign country? International airports are "neutral ground" so as long as you stay on the "international" side, no visa is required.

@Jpe61: That is too simple. You might not need a visa, or you might need a simpler "transit" visa, or just an ordinary visa - countries are free to make their own rules, and those could depend on the nationality of the pilot. I'd imagine that quite a few countries would ban El Al pilots altogether.

Note: it is not a duplicate. The "duplicate question" is about not leaving the plane (or the airport). From the comment of OP in the answer, it seems OP want to do touristy things, so a different case. @Encipher: maybe you should add that information in the question. In any case. I'm voting to reopen the question.

@GiacomoCatenazzi Yes exactly I want to know how much they have access over the country as a tourist. I wonder why people of stack exchange are so interested to downvote a question, flag a question as duplicate, and even closed questions. I believe questions open our minds.

You question is not so clear. My interpretation was based on your comment in an answer. So, good question, but you didn't write it well. I think you should edit your question, adding some details of purpose of entering a country, for how long. [but to answer: your airline will tell you what to do, and they look for the permissions (you cannot flight back the airplane: required rest time). It is more complex for VIP pilots (they must figure out themselves)]

International Airline crews often spend 1 or 2 days laying over at the destination. In most countries, airline crew members who are listed on the Aircraft Customs and Immigration General Declaration are exempt from Visa requirements.
Rules will vary from country to country as to how much travelling freedom you have while staying in the layover country. Most countries require you to always have your passport with you and some countries require some kind of form identifying you as airline crew. Some countries have rules defining how far you can travel while on the layover.
In general practice most airline crews I know leave their passports in their room and just carry a photo copy or digital copy with them, and freely travel just as any tourist would do.

Are they eligible for country tour, buying souvenirs in short are they eligible to act like a tourist?

I edited my answer with more info to answer your question.

Other countries, like the US, have specific visa categories for plane or boat crew members (in the US e.g. C-1D).

Thank you Mike for the detailed reply.


text:
When does the FMS in the 737NG switch to approach mode?
I fly a B737-800, and we talk about when the FMS enters "Approach mode". At this point, the FMS will command the Vref (plus additive) at the appropriate flap setting. This is helpful to know especially when there is a flap malfunction.
When does the FMS switch to "approach mode" for calculating speeds, and where do I find this information? I haven't seen it in the FCOM.

If the 737 is like most other FMS installations you will find the approach will activate at the capture fix.  It is typically the waypoint just before the FAF.  If you were flying into KMEM RNAV RWY 36C the capture fix would be NESBT and the FAF would be GINIE.
In the FMS that I use, the capture fix is typically loaded twice into the flightplan.  Once from the enroute structure and once again for the approach after the approach name.

NESBT
** RNAV 36C **
NESBT
GINIE

The FMC transitions to “on approach” when the airplane is within:

2 NM of the first approach waypoint (including approach transitions such
as arcs and procedure turns), or
2000 feet of airport elevation, whichever occurs first.

(Boeing 737 NG FCOMv2 11.31.37 - Flight Management, Navigation - Flight Management System Operation)


text:
If an airliner turns back midway, how is the route planned?
There's several incidents of "flights to nowhere", e.g. https://onemileatatime.com/klm-787-nine-hour-flight-to-nowhere/
After a pilot decides to turn back, how is this handled from a routing/flight plan/ATC perspective?  I assume ATC needs a flight plan back to the airport; is this just generated "on the fly"?  Who does this, ATC or the airline's dispatch?
I'm not talking about a diversion to the nearest airport but actually returning to the origin airport after the flight's been underway for a while.  I assume this needs to be coordinated with all the FIRs/centers.

Why do you think it might be handled differently than any other unplanned enroute diversion?  You'd simply request clearance back to your origin.

I guess that's how it would work in theory, but say if I'm halfway over the Atlantic, would ATC actually give a clearance back to the origin and would they plan the route on my behalf?

It also depends on why they turn back. If they declare an emergency, they're allowed to do pretty much anything.

ATC doesn't flight plan for the crew.  The captain would likely request direct, or via a fix or two presuming that they are familiar with the route.  ATC will generally clear them as requested, or may modify as needed for separation or procedural requirements.  And the clearance limit would likely be the origin airport.

A diversion is a diversion, regardless of whether it is to the nearest airport, back to your origin airport or some other airport that is attractive, which may depend on what sort of facilities are needed (gates, fuel, maintenance, etc.).
The pilot simply tells ATC their new destination, and ATC amends their flight plan and gives the pilot a new clearance to follow. Depending on how far it is, the pilot (with the help of their dispatcher, if applicable) may request a specific route, or they may leave it up to ATC. That flight plan update is also automatically forwarded to all the relevant FIRs, just like the original version was.

How would ATC determine the detailed route? It's not like we know every waypoint and airway in every country by heart, let alone all the different usage restrictions. And how would it be communicated? - Dedicated ATFN FPL (APL, ACH, ...?) telegram or coordinated manually?

@expeditedescent Is this the one area where the FAA is actually ahead of the rest of the world? When we file a flight plan, either on the ground or in the air, we can request whatever we want (such as “direct destination”) and the FAA computers automatically “fix” it to comply with all applicable rules, then ATC gives us back the resulting clearance (either CPDLC or voice).

Sure, if you're staying within the same country (or maybe just wants to cross a single border) - no problem. But if a long haul flight has to turn around half way, crossing through several countries on the way, I doubt a single click of a button would be enough. Maybe not a very common problem in the USA since you do not border a lot of countries?

@expeditedescent Considering our vast OCAs, including a large chunk of the Caribbean, the US borders far more countries (or at least FIRs) than it may appear.

Having done a turn back over the Indian ocean back to middle east, I used both my knowledge of the surrounding sectors, another aircraft that was heading to the same region and a world airways chart that was available with our flight data coordinators (they manage the ATC data in the system). This resulted in a not perfect route but one that got them to there desitnation. This is picked up by the down stream controllers and corrected as it gets to them. As my instructor once said, track direct destination is still a legal instruction to get them going in the right direction.

If you, as an airliner, make a midflight request to return to your departure airport, we will get you your clearance.
This would be a very unusual request, though, and you can expect some follow-up questions from ATC, regarding the reason for turning around. (probably some prompting from a supervisor in the background)
If it is not an urgent situation, you can expect a normal arrival routing to your new destination.
That may not be the end of it. You should expect further clarification being required, since you are hauling passengers for hire. This may be handled by your company, of course.


text:
When was the last time the US Standard Atmosphere 1976 was updated?
I imagine due to slight changes in Earth's atmosphere or more accurate instrumentation it's been updated at least once, but also because it's understood to be a less precise model we might not have even bothered. I haven't been able to find any evidence/papers so far detailing the changes, however.

Welcome to aviation.SE! If you don’t get a useful answer here, you might try asking on earthscience.SE.

No, this model was last updated in 1976, and only differs from the 1966 model above 51km in altitude, where aircraft rarely fly anyway. There are no newer versions of this atmospheric model.
There are other models though, some of which may be newer.


text:
How do you know if an aircraft engine has a supercharger?
The engine I'm specifically thinking of is for the C-130J Hercules, it's the AE-2100 D3.

As Bianfable has pointed out, the C-130 Hercules does not have a supercharger or a turbocharger. No turbine-powered aircraft have them. Turbine engines do not need these forced-induction chargers since turbine engines compress the intake air themselves. It would be impractical to design a supercharger or turbocharger for them.
The easiest way to know if a piston-engined aircraft has a supercharger or a turbocharger is to check the aircraft’s POH. If one is not handy, you can look at the aircraft engine itself.
An engine with a supercharger will have an accessory belt (usually wider than the other accessory belts) attached to the crankshaft or flywheel. In older aircraft (WWII, Korean War, etc), the supercharger will be less obvious. These may even be chain or gear-driven. The application of superchargers in this case would be for large, powerful engines.
An engine with a turbocharger will have the exhaust manifolds converge down to one pipe. That pipe will feed into the exhaust turbine of the turbocharger. Although, the exhaust turbine might not be readily visible, the housing for the compressor impeller should be more obvious and may obscure the turbine. What would be more obvious would be the additional manifold piping and ducting.
If neither of the above are accessible, just look at the model name of the aircraft. Most modern small General Aviation aircraft manufacturers will want to advertise that their model is turbocharged. A manifold pressure gauge that goes beyond 30 inches of Mercury may be an indicator as well.

Thank you for all your help everyone!

Do you know if turboprop engines have a critical altitude like engines with a supercharger?

@BillShao - To be clear, there is a big difference between a turboprop engine and a turbocharged engine with a propeller. Terminology is important here. Turbine engines can be classified as turbojet, turbofan, turboprop, or turboshaft. Piston engines can be classified as naturally-aspirated, supercharged, or turbocharged with a further classification of carbureted versus fuel-injected. The term “Critical Altitude” is most commonly used with force-induction engines like ones with superchargers and turbochargers. It is generally not used with turbine engines.

Superchargers and turbochargers can be used on piston engines to increase the manifold pressure above ambient pressure, meaning more air can go into the cylinder during each stroke. This increases the power output of the engine. While superchargers were quite common in high performance aircraft of the past, they are relatively rare today. Most aircraft piston engines today are naturally aspirated or turbocharged:

Back in the mid-1900s, and especially during WWII, superchargers were usually the engine boost of choice. Their simpler operation was one of the main reasons. Another reason they were popular was a lack of metals that could handle the heat produced by turbos. Gas was also very cheap, so it wasn't that big of a deal to burn a little more gas to get the power output you wanted.
High temperature alloys, lighter weights, and better fuel efficiency have all made turbos the best choice for almost all of today's aircraft. And, with automatic waste gates on many models, they're easier (and more foolproof) to operate than ever before.

(boldmethod - What's The Difference Between Turbochargers and Superchargers?)
The Rolls-Royce AE 2100 engines used on the C-130J Hercules are turboprop engines. They don't have any pistons and therefore no super- or turbochargers. A turboprop engine looks something like this:

(Wikimedia)
The air entering the engine goes through a series of compressor stages, which increase the pressure, before going into the combustion chamber. This is more equivalent to the compression stroke of the piston engine, but like a supercharger this compressor is powered by the engine itself (it is on the same shaft as the turbine). Every turbine engine has such a compressor, but it is never called a supercharger.

To answer your title question, how do you know if an aircraft engine has a supercharger? For a piston engine, it will be specifically mentioned, since it is relatively special. But for a turbine engine: never.

Thank you for the information!

I've read that the reason for turboprop engines is that after WWII, research was done to find the optimum size of turbocharger for a piston engine.  Turned out the optimum was to delete the piston engine entirely.


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How many zig-zag patterns are there in a dog tooth leading edge?
And also what is the difference between saw tooth and dog tooth leading edge?
Like is there any difference in the number of "zig-zag" patterns on the wing?

Hi and welcome to Aviation.SE Feel free to take the tour https://aviation.stackexchange.com/tour Also you might want to consider adding pictures or links. And maybe dont ask different questions in title and body

Both terms (“zig-zag” or “dogtooth”) designate the same leading edge aerodynamic design, and are used interchangeably. “Saw-tooth” is also used to indicate the same leading edge pattern, yet it is also used in naming a specific trailing edge design (a “saw-tooth” trailing edge; see this paper which explores this effect: Saw-tooth, or, Serrated trailing edge
The number of “cuts” (or, extrusions of the leading edge of a certain wing section compared to the leading edge towards the root of the wing) are a choice of design.
The “dogtooth” (or “sawtooth”, or “zig-zag”) at the leading edge acts in a way similar to a wing fence ), inducing an effect on the relative airflow which acts to prevent span-wise flow (and also adds energy to the air flowing over any control surface in the area, increasing its effectiveness.
As there are aircraft with numerous wing fences, so can the dog-tooth extrusion be multiplied on each wing, however, it is a question whether there were any planes out there with more than three such points of extrusion.
See this question for more detail: Hawker Hunter Dog-tooth wing - Aviation SE
Take a look at these pictures: notice the “dog-tooth” / “saw-tooth” leading edge and “saw-tooth” / “serrated” trailing edge.


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Compressor variable geometry and Engine pressure ratio
I was wondering does a compressor variable geometry and a inlet variable stator vane share any similarities or does these terms have same meanings?
I did some digging and all I could find is that CVG is most applicable to turbochargers like the ones you'd find on cars.
However, the term 'CVG scheduling and control' was mentioned as part of a FADEC control output in my gas turbine engine module.
It states that "the FADEC stores a schedule of CVG position versus corrected N2 so as to achieve optimum compressor efficiency. CVG position feedback fed from the CVG actuator on the engine is compared with the CVG demand to achieve a closed loop control. Thus, the inlet guide vanes and the first 5 stages of the CVG are controlled"
This statement rises another question because I do understand that a inlet variable stator vane can aid to change the angle of the incoming air which is passing through the engine and therefore it can manipulate the pressure which is at the inlet and this VSV are controlled by actuators (purposely to minimise compressor stalling effect).
Now how does the CVG fit into this context, is it a system that is solely utilized by the FADEC in order optimise the AOA of the IVGVs? And if so, why is that if the total engine pressure exceeds its limit(which I know that in such condition the EGT has increased), the CVG would be turned off to compensate for air control. Shouldn't the Fuel control unit decrease the flow of fuel into the burners?
I know these is quite a mouthful but if any one can answer or at least shine some light I'd appreciate it.

Welcome to Aviation Stack Exchange. Please limit your question to a single question, without any "in addition..." secondary questions. Also, please specify which engine your question refers to; the mechanism you're referring to isn't universal.

I haven't made an attempt as yet to find which engine has this mechanism but it was during our gas turbine engine module(aircraft engine) especially when we were digging into FADEC I came across this term 'CVG'. This question came out from EASA examation and I wanted to know if anyone would know the difference with the variable stator vanes and the CVG and how CVG and the total engine pressure operates.The question stated that if the engine pressure ratio exceeds its limit value what would happend? And the correct answer was that the CVG system would close inorder to compensate for airflow

Do you have any reason to think compressor variable geometry is not the generic wording encompassing all features used to adjust the compressor geometry and remain at the engine design point: VIGV, VSV, VBV? See this thesis, e.g page 24.

Ralph, I can't thank you enough for the information. Upon reading it and reflecting on this diagram I have with me(how the FADEC receives inputs and then sends one of its output to the CVG in order to compute the optimal performance of an aircraft) I am convinced this CVG is a system that FADEC uses to manipulate the  VIGVs. Thank you much


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How to know how much overshoot/undershoot the compass if you're flying close to one of the poles; is it even needed?
As far as I understand, the formula (1/2LAT + 15°) is not applicable when flying close to one of the poles or the equator.Therefore, when flying close to the poles, what happens? Does it have the same effect as when you're at the equator? Does the magnetic dip become insignificant enough so that the compass lag/leading may be neglected?

Are you talking about VFR or IFR?  If IFR with no directional gyro, you normally do timed turns.

If you’re flying between the North Pole and magnetic north, the two will differ by 180 degrees.  Also, the magnetic South Pole isn’t diametrically opposed to magnetic north; there’s no simple formula

If you're flying really close to the magnetic poles, the compass becomes fairly useless anyway.  In the far north you fly by true headings and tracks.

As far as I understand, the formula (1/2LAT + 15°) is not applicable
when flying close to one of the poles or the equator.Therefore, when
flying close to the poles, what happens? Does it have the same effect
as when you're at the equator? Does the magnetic dip become
insignificant enough so that the compass lag/leading may be neglected?

Re the question in the last sentence quoted above: definitely not.  Magnetic dip is actually more pronounced near the magnetic poles than anywhere else on earth.
See for example http://geokov.com/education/magnetic-declination-inclination.aspx -- the second of the two charts shows magnetic "inclination", or "dip".
A good visualization of the strong "dip" in the earth's magnetic field lines near the magnetic poles is provided on this web page.
The strong "dip" in the earth's magnetic field lines near the magnetic poles is one of the main reasons why it is so difficult to use a magnetic compass near the magnetic poles.


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What is this double-decker jet that Qantas flew—or did it?
This is a screenshot from one of Petter's mentour pilot videos:

I thought two-floor passenger jets were impossible because they couldn't be evacuated in time.
I'm wondering if his editor accidentally included a shot of a simulated future aircraft.

There are numerous clips of actual double deck A380s in flight, and on the ground in the video you mentioned. Do you not believe the aircraft shown in all those clips are real?

thought two-floor passenger gets were impossible because they couldn't be evacuated in time

The video of the evacuation done for certification is available online and quite impressive since it must be done within 90 seconds.

What is this double-decker jet that Qantas flew—or did it?

Not only Qantas but also All Nippon Airways, British Airways, Emirates, Etihad Airways, Korean Air, Lufthansa, Qatar Airways and Singapore Airlines for a total 251 aircraft (half of that only for Emirates).
The A380 has actually been a very nice example of "the customer is always right" and that you should listen more carefully to their needs when you plan to build something for them (except maybe if you are Apple). The whole story is very well described in the Wikipedia entry.

... if you are Apple, or French. FTFY.

The screenshot depicts an Airbus A380. The A380 is notable (as the first paragraph of the Wiki article says) for being the only full-length double-deck jet airliner ever produced.
Qantas operates ten A380s, although they are slated to be replaced by the single-deck A350s beginning in the next decade.

Any idea who will be getting the double-deckers?  Or will they be scrapped?


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What is this strake near the nose of the MD-80 for?
What is this for on the MD-80?
Is it a stabilizer?

Aside - these are analogous in function to a Skeg or Bilge Keel under a ship intended for water.

TLDR:
They are there to improve directional stability.
At high angles of attack, the wake from the fuselage covers the rudder, reducing yaw authority. Strakes mitigate this effect and thereby improve the directional stability.
More information:
This paper from NASA/Boeing contains more detailed information.
An image from the paper indicates the difference clearly:

$C_{n_\beta}$ denotes the correcting moment yawing $C_n$ for when a given yaw angle $\beta$ is present.
As the image shows, it needs to be larger than zero for an aircraft to be stable. Strakes help to keep $C_{n_\beta}$ positive at high angles of attack.
$C_{n_\beta}$ is usually delivered by the rudder, which at large angles of attack gets blanketed by the wake coming off the fuselage, as shown in the image below:

Source
Once the rudder gets blanketed by low energy flow emanating from the fuselage, it becomes less effective and has greater difficulties straightening the aircraft into the wind.
Strakes mitigate this effect and thereby improve the directional stability. The working principle is similar to vortex generators, see this question, and others on the same topic here on Aviation Stackexchange for more info on vortex generators.
The paper also shows a benefit in lateral stability (i.e. in roll), but this effect is minor (and does not change the aircraft from unstable to stable).

Interesting to see that Boeing published a study in 1998 to understand what Douglas had done in the early 70s. At thetime, Boeing just bought McDonnell Douglas and inherited the MD product line, but it lacked detailed engineering documentation that Boeing was used to. Boeing at the time was still very much an engineering company, while MD was run by accountants, financial controllers and other managers obsessed with cost savings.

Unfortunately the MD management culture became dominant after the merger. Over the next 20 year there's been less budget for this kind of research, up to the point where Boeing aircraft (or pieces thereof) started falling out of sky .

This work was done by Boeing for the STRATA-project (Strake Technology Research Application to Transport Aircraft) which was a NASA/Boeing collaboration. So it could also be that they recreated earlier findings to bring it in the public domain (for example by using non-proprietary geometry etc.)

The nose strakes on the DC9-50 and subsequent models are to energise the airflow around the fuselage at high angles of attack.
Without the strakes, at high angles of attack, the turbulent flow around the fuselage interacts with the vertical stabiliser, reducing the directional stability. The strakes introduce a vortex into the flow, ensuring the flow contains more energy which in turn improves the directional stability at high angles of attack.
The shorter models didn't need these strakes since the shorter fuselage doesn't cause so much turbulent flow over the vertical fin.

I'm sure they also help to improve spin characteristics, but for airliners directional stability is clearly more important. +1

What is angle of attack?  (And which direction is high/positive ?)  (Or does high angle of attack mean the absolute value (of whatever angle of attack is) is large?)

The question which arises is then... how do other airliners which do not have these strakes do without?

@jcaron, these devices usually get added as late-stage development mitigation device. For example, when during certification flights a deficiency gets discovered. In this case, the aircraft was a derivative (a lengthened version of the DC-9-10), and the strakes were a quick fix to prevent a more radical redesign.

@jcaron also as @ DeltaLima said: "The shorter models didn't need these strakes since the shorter fuselage doesn't cause so much turbulent flow over the vertical fin." The wings and engine are very to the back and is a long aircraft. I also noticed these strake on the Concorde which is similar case(wings and engine to the very rear of the aircraft)


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Why is this flight from Morocco to Saudi Arabia flying such a long detour?
I saw the following flight path for RAM2014 from Oujda to Medina by accident on Flightradar24. Why is it doing such a long detour to go to Medina?

I'll wait for the proper answer but usually that is because the intervening countries simply don't allow the plane and/or the airline through. Or the airline doesn't feel safe being in their airspace.

This is due to the closure of the airspace, as ratchet freak indicated.
I could find news reports from 2015 saying the airspace was closed:

Algeria closes airspace to Marocco, dated 2015
Marocco stops flights to Libya and closes airspace (likely reciprocated), dated 2015

Concerning Libya Al Jazeera indicates: "The Arab Spring and Gaddafi’s overthrow has made the security situation in Libya profoundly complicated"
However, it seems the situation is improving, as there are talks of opening flights between Marocco and Libya

Flight path from FlightAware with labeled FIRs

I believe there is a feature to see the airspace in FlightRadar24. If someone here has it, they are welcomed to add a picture of the Flightpath on top of the airspace borders

@fooot thanks!!


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What is this biplane seen over Doncaster?
This plane passed over our house in Doncaster England and we’d like to know more about it please:

This one is decidedly easy to answer, a modern photo clearly showing the aircraft registration means you can search the UK database of registered aircraft.
G-EMNN is a FOCKE WULF FW44J built in 1936, and previously registered in Germany as D-EMNN.

You can also use the horizontal tail for identification. The FW-44 tail shape is like a fingerprint.

You might find this airplane's flight history on  https://www.flightradar24.com/data/aircraft/g-emnn


text:
What airplane is this? Seems like a Douglas?
What is this airplane? I flew to Homestead General Aviation airport (X51), and at the ramp (Homestead Executive Jet Center) I saw this aircraft:

I thought this might be a Douglas DC aircraft but I have no idea what it exactly is.

Convair 240 N99380 built as a T-29B for the United States Air Force with s/n 51-5118.

Are you asking about the big white or the small grey airplane?

It looks like a Convair CV-240.
"The Convair CV-240 is an American airliner that Convair manufactured from 1947 to 1954, initially as a possible replacement for the ubiquitous Douglas DC-3. Featuring a more modern design with cabin pressurization, the 240 series made some inroads as a commercial airliner, and had a long development cycle that produced various civil and military variants. Though reduced in numbers by attrition, various forms of the "Convairliners" continue to fly in the 21st century".


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What if the onboard airspeed sensors fail?
I've a simple question regarding the rare case of the failure of all airspeed indicators (ASI) including the co-pilots's indicator. Say the airspeed indicators were to fail during rotation, what would be the standard procedure then?

The answer depends a great deal on the aircraft.

Details vary but I would think regardless of aircraft type you would continue the takeoff and climbout using pitch and power as your cues until you're able to regain reliable and accurate airspeed indications or just stay in the pattern and come back and land. Angle of attack indications would be nice but not required.

This question is related: https://aviation.stackexchange.com/questions/105181/can-a-piece-of-duct-tape-bring-down-a-plane-today-flight-603-1996-per%c3%ba and so is this: https://en.wikipedia.org/wiki/Birgenair_Flight_301

I think it depends a lot if it happens on a Cessna 172 in the daytime or on a jetliner at night on the ocean

In a light aircraft we're taught "pitch plus power equals performance" - and to an extent the same is true from a tiny Cessna up to a giant Airbus.
My instrument instructor reminded me that I neither need an RPM gauge nor a speedometer to know when to change gear in a car (yes we used to drive manual cars!) as you just could feel, with some experience, when was the right time to do so.
This holds true in a light aircraft for airspeed, with some experience you can feel the speed - especially approaching a stall as the controls start to get "mushy" a good 5-10 knots before the stall.
So taking off with an inoperative ASI is fairly benign. Power is typically at or near full, and you pitch as you normally would for a climb. Pitch + Power = performance.
Landing is slightly more work, its still pitch + power, but it certainly helps to have a good amount of experience of that type so you can feel if you start to get too slow. When I tried it with an instructor who had covered my ASI he asked me on final what speed I thought I was doing, and I correctly guessed within 5 knots. It was the absolute normal approach speed I had done a hundred times before.

Step 1 in any emergency: Fly the airplane.
Thrust and pitch control your airspeed. A good pilot knows the settings for each phase of flight. It may not be exact, but it will keep the airplane flying.
Step 2: Determine the problem. Refer to checklist.
The crew alerting system will tell you if there are any detected faults and in which system. For Airspeed, you can also cross check between CAPT and FO and with IRS/GPS ground speed. And in an air transport aircraft, there's a third air data unit (sometimes AD3 or Standby or SAARU).
Step 3: Take corrective action.
Declare an emergency. Fly to an safe altitude and area to determine next actions. Prepare for a landing as soon as practical (May require dumping fuel or alternate airport.)

Thank you for your reply!, I found it quite helpful.

@Aerospace_Nerd Welcome to Aviation.SE! Instead of commenting thanks, please consider up-voting and possibly accepting the answer. Have a look at the tour to see how the site works.


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What is this phenomenon where a narrow stream of airflow off a nacelle becomes visible?
I few hours ago I was on an A321neo going across states. As we were approaching and descending to the runway, I saw this cool effect near the engine and wing fixture:

If you look near the engine, you will see airflow. But why is it condensed near that spot specifically? Why not anywhere else? What is the physics and aerodynamics between such a phenomenon?

Related (including more cool pictures): What is this flange on the engine?

It's basically an artificially generated tornado laying on its side. As with an upright tornado, you see it because of condensation of water vapour due to the pressure drop in the core.

Just wanted to add that I saw this recently as well, thought it was pretty neat!  My pictures and a quick video: https://imgur.com/a/cGHAk17

It's the visible vortex created by the fan cowl strake at the engine of the A320
Below is an image of the same on a Boeing 737,

On jet aircraft where the engines are mounted in nacelles slung under the wings, strakes may be added to one or both sides of each nacelle to produce vortices that energize the airflow over the wings in times of high angle of attack, such as during takeoff and landing, thus improving wing effectiveness.
Sometimes due to humidity and other atmospheric conditions the vortex generated by these is visible prominently.
Some more info about these from R.S. Shevell. Aerodynamic Bugs: Can CFD spray them away? 1985

DC-10 wind tunnel tests showed a significant loss in maximum lift coefficient in the flap deflected configurations, with landing slat extension, compared to predictions. This resulted in a stall speed increase of about 5 knots in the approach configuration. The initial wing stall occured behind the nacelles and forward of the inboard ailerons. The problem was traced by flow visualization techniques to the effects of the nacelle wake at high angles of attack and the absence of the slat in the vicinity of the nacelle pylons. The solution was developed in the NASA Ames Research Center 12 ft. pressurized tunnel and turned out to be a pair of strakes mounted forward on each side of the nacelles in planes about 45 degrees above the horizontal. The final strake shape was optimized in flight tests. The strakes are simply large vortex generators. The vortices mix the nacelle boundary layer air with the free stream and reduce the momentum loss in the wake. The vortices then pass just over the upper surface of the wing, continuing this mixing process. The counterrotating vortices also create a downwash over the wing region unprotected by the slat, further reducing the premature stall. The effect of the strakes is to reduce the required takeoff and landing field lengths by about 6%, a very large effect.

Related:

What is this flange on the engine?
In what conditions are visible Wingtip vortices created?
Can these vortices be dangerous?


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Takeoff distances published in OEM manuals
Background
For academic research purposes, I have developed a first-principle takeoff performance model that returns a takeoff distance required (TODR) based on the aircraft type (incl. its associated geometric and propulsive characteristics), takeoff mass, and environmental conditions.
The model first calculates the stall speed based on weight, air density, wing surface area, and a lift coefficient in takeoff configuration, and returns 120% of that stall speed as an estimated safe liftoff speed.
For each airspeed interval up to the liftoff speed, the model then calculates the dynamic pressure, thrust, acceleration, and horizontal distance under all-engines-operating (AEO) conditions. The model then returns 115% of that distance as TODR, in line with regulations (14 CFR § 25.113 and CS-25.113). The model deliberately ignores the accelerate-stop distance (ASD) and the accelerate-go distance (AGD) for now, although they may actually be greater than 115% of the AEO distance.
I am calibrating the model (using the lift coefficient as the adjustment variable) for two narrowbody aircraft (A320 neo, B737 Max 9) and two widebody aircraft (A350-900, B787-9). To do so, I use an online plot digitizer to interpolate OEM takeoff performance data (at sea-level ISA conditions) published under the respective "Aircraft Characteristics Airport and Maintenance Planning" (Airbus) and "Airplane Characteristics for Airport Planning" (Boeing) reference documents for each aircraft. The charts in Appendix below are taken from those four documents and provided for easier reference.

Questions
I have three questions about these charts, so that I can make the right calibration assumptions:

Is it reasonable to assume that these charts indicate the horizontal distance traveled by the aircraft up to reaching the screen height of 35 feet, and not just up to liftoff from the runway?

Is the takeoff distance indicated on these charts the regulatory takeoff distance (i.e., the greater of 115% of the AEO distance, ASD, and AGD), or just the actual/physical measured horizontal distance (to either screen height or liftoff, depending on your answer to Q1)?

Is it reasonable to assume that the takeoff distances indicated on these charts were measured using full thrust / TOGA power (i.e., not derated nor assumed temperature / FLEX thrust)?

Appendix - Charts
A320 neo

A350-900

B737 Max 9

B787-9

Three distances should be calculated, and the longest one taken as the TO distance: the distance to clear 35 ft of terrain with engine failure, distance with full engine power * 1.15, and the emergency braking distance. More background in this answer.
So to answer your questions:

The indicated TO distance is the greatest of the three possible cases. Indeed to clear 35 ft of local terrain after engine failure above V$_1$.
The charts show the regulatory TO distance indeed.
Regulatory TO distance is shown at full TO power.

Thank you very much, @Koyovis. I marked yours as the accepted answer. I ran the calibration with these assumptions and am getting average lift coefficients ranging from 1.93 and 2.43, which I think is plausible considering that Roskam Part I recommends values of 1.6-2.2 for turbofan transport aircraft.

@teedeepee Some additional data on $C_{Lmax}$ in this answer

Thank you, @Koyovis. That's helpful as a proxy, too.
I'm also aware that Eurocontrol's BADA has CLmax values for more modern airframes, but unfortunately they won't disclose their data easily and have a restrictive licensing model.


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Are these questions really on the FAA Glider Commercial written test?
I'm currently working on my Commercial Glider rating and for the written, I am using a popular test prep package.  The package allows the specific selection of Commercial Glider (vs Airplane, Helicopter, etc.)  Indeed, their glider question bank has 639 questions, whereas the Airplane and Helicopter question banks have 881 and 727 questions respectively.
So I was surprised to be presented with several questions that I wouldn't have expected glider only pilots to have to know.  For example:

I am also a PPSEL, so I was able to answer this, but I've never come across a glider with a VOR installed, and I frankly am not sure how useful one would be.
I was even more surprised to find this question:

Why would this be on any Commercial written test (let alone the Glider commercial?).  An instrument rating is NOT required for any commercial rating AFAIK, and certainly not for a Glider commercial rating.
So my question is, is this just a mistake in the test prep software, or are these types of questions actually on the Glider Commercial written test?

As for "Why would this be on any Commercial written test", the FAA commercial ratings for powered aircraft (airplanes, helicopters, gyros, powered lift vehicles, and airships) do not require instrument ratings but they do require a certain amount of instrument training.   For gliders I agree it doesn't make much sense.

@Brian: Even if you're looking to fly an IMC-certificated glider?

I wasn't aware there was such a thing.
That said, wouldn't you need an instrument rating to fly in IMC?  My question is really two pronged, 1) Why on a commercial test (for any category) as opposed to a Instrument rating test and 2) Why on a glider test.

Well, now that I've taken (and passed) the test, I feel somewhat qualified to answer my own question.
The first question, The VOR CDI indication deflection question, was indeed on the test.  There was also a question on VOR receiver checks "from a designated checkpoint on the airport surface".
The second (IFR) question was NOT on the test.
Of course, the test has 100 questions and the question bank is reported to have > 600 questions, so there is no garuntee that it couldn't be on the someone else's test.  IOW, YMMV.

Interesting.  Well, the main thing to know about flying gliders in IMC is that the yaw string might freeze to the canopy as the canopy ices up, so it's good to have a slip-skid ball on the panel.  Now, how would I possibly know that?  :)

Thanks for coming back to answer your question!

Congratulations on passing your test!

There are various types of aircraft that qualify as gliders. I've seen them with no engine, sustainer engines of various types, self-launch capability and even with large turboprop engines. (A little trivia -- the FAA does not define what is a glider in the experimental category...) Gliders also come equipped with various equipment, from the basics of VFR to full IMC instrumentation and avionics.
Because of this you can find "gliders" quite capable of flying VFR and IFR. I am not surprised by the VOR question.
IMO, you have a valid point in the second part of your question. The only explanation I can think of is that the FAA apparently does not remove questions from the commercial pilot plain vanilla database for the other exams, but rather adds questions related to gliders, for example, and requires that the test include a certain number or percentage of questions from that group. Also, the FAA doesn't have a commercial pilot instrument rating test. That said, neither of these are explanations for what you have pointed out.


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What is the optimum viewing angle to search for something on the ground from the air?
Continuing my series on our upcoming search for a downed Douglas B-18 bomber in Alaska* (13 more days to go!), I've started to think about the angle we should use to "look down" at the terrain.  This is especially important because I will be mounting go-pro like cameras on the airplane to capture what we "see" on the ground as we go.  I will do this with this handy tie-down mount I came across:

This mount allows the camera's to be positioned at any downward angle.  I wouldn't think that either straight down or straight ahead would be optimum...I would assume a good angle would be somewhere in between.
Obviously this question has some relation to photography, and I may also post this question to Photography.SE.  However, I would also assume there are a great number of pilots and other aviation professionals with experience searching from airplanes/helicopters for things on the ground.
So my question is, what is the best angle (relative to straight ahead) to search for something on the ground?
Keep in mind that I will be searching over glaciers around an active volcano in partially snow covered terrain.
* Related posts:
Where can I find more information about my grandfather's crashed B-18a bomber on Mt. Redoubt Alaska? and
What navigational aid(s) would WWII bomber pilots have used while flying in instrument conditions?

Straight Down is most likely the best angle but there is of course cases to be made for other scenarios. In your case if you are looking for something on the ground that is potentially obstructed a straight down view is going to give you the most direct view and will prevent something from falling out of frame if an angled view cause an obstruction to block it.
The real problem in areal viewing/photography/imaging is the air its self. They touch upon it in this podcast nicely but when it comes to spy satellites (a similar use case) the actual resolvable resolution was limited by the air not the imaging sensor. Its important to go up on a clear day as haze, even VFR legal haze can have a big impact on this.
If you are intending on using a GoPro I might advise getting something with a longer lens (especially if you intend to be at any kind of substantial altitude). For comparison you can check out some of the stuff I have captured with a GoPro here none of that footage was shot over 5000 and most of it below 3000 and it is difficult to pick out smaller objects on the ground in the footage. At the flat mounting angle I use you can see how trees obscure things behind them.
As for mounting bigger lenses, its clearly impractical to point large lenses straight down but there is a work around. Historically spy planes used some kind of mirror set up to look straight down at the ground mounting the large lenses flat in the plane pointed at a mirror.
There are even some smaller GA planes built for this specific purpose with cameras already mounted. Some may simply have a camera trap door. It may be worth it to try and track down this kind of service for your mission.
Your lens choice will also be quite important. The GoPro has a fairly wide lens which offers a great big field of view but considerable edge distortion in actual practice at longer distances. I would advise trying to rent or borrow (unless you own) a decent DSLR with a standard to short-tele lens. On top of that I would not video tape the adventure, I would set the camera to take a series of time laps photos pointed straight down. Most modern cameras can resolve a much higher res image photo than they can video and in the end of the day at a modern frame rate you are going to end up with a log of overkill footage at your given speed. This is how planes like the SR-71 did it back in the day. In this case a decent GPS enabled camera is key, remember you are looking through this data after the fact so you need to know where the picture was actually taken to make it useful.
You also need to think about field of view a bit. The wider the lens the more land you can cover in a single pass at a given altitude but you do so at the cost of angular resolution. The longer the lens the smaller field of view with the benefit of higher angular resolution.
Note: You will find some info out on the web about the use of wide angle lenses in areal imaging. The common ones were the likes of the Zeiss Biogon and Distagon mated to either a custom platform or in the case of the moon missions a Hasselblad body these particular lenses are phenomenally sharp in the edges even wide open. They are however the exception to the rule and most modern wide lenses make trade offs to be compact and light weight. To be safe I would stay away from wide lenses (anything bigger than maybe 28mm on your standard Full Frame SLR) for your trip.
One fairly reasonable option is the variety of mirror reflexive telescope/tele-photo lenses out there. Something like this one could be useful in your endeavor. A few of these have passed through my collection and they are far from great but they do get the job done when you need a big lens in a small package. However I would take a used lens like this one which can be had for the same price any day.

For the sake of a complete answer Im going to add more details although in your specific case they may not apply.
The most successful areal imaging programs quickly adopted stereo imaging setups. In this case 2 cameras were used each mounted slightly off axis of centerline. The Corona program as well as the KH-7 Gambit satellites used such a system with great success. The images when viewed later created a somewhat 3D rendering of the ground which provided superior analysis abilities over the more traditional single camera units. This kind of effect could be achieved with 2 cameras mounted on some kind of bar and a little pre calibration.

The relative angle of the Sun's direction is very important, too...

@xxavier can you elaborate?

To put an example; specially when flying low, the accidents on the ground are difficult to see if the visual and the observer-Sun direction form a small angle, and much easier to see with a bigger angle. An extreme case (if not precisely of low level flying...) is the appearance of the Moon's craters, almost invisible at full Moon, when the above-mentioned angle is small, and easily visible with a crescent Moon, when that angle is large...

The higher quality DSLR equipment mentioned is eye-wateringly expensive to buy (though cheap in comparison to plane parts, I'm sure). You can rent high quality equipment for comparatively little, though. Recommendations are typically off topic, so I'll just mention Camera Lens Rentals and Borrow Lenses as places I've used in the past. They also offer high quality, dedicated video equipment, as well. In your situation, I'd opt for purchasing the rental insurance - just in case... ;)

So... longer shadows makes things easier to see?  Is that what all that means?

@Anyone with experience in flying low, specially in helicopters or in very slow planes knows that there are times of the day and Sun orientations that make it very easy to see objects on the ground, and times of the day and Sun orientations that make that very difficult. The length of the shadows is a factor, but by no means the only one.

@Dave thanks for the detailed answer, will review today as you've provided quite a bit of info.  I've also subscribed to the podcast you mentioned.

Im gonna add some more (for the sake of info) but the rental id vice in the comments here is solid. Take a look at a Nikon D3300 and an Ai-s 300mm full manual lens. Nice set up for a very reasonable price.

I spent an enormous amount of my childhood & early teens looking out the window of my father's Cessna 337. I found that to look as close to vertically down as possible is actually quite unnatural, the eye will unconsciously keep fixating on details & then whip-panning to the next random detail. The fovea (the surprisingly small area of decent vision our eye has) is designed to continuously move around building up an image & correcting what the brain thought it saw.
Our image of the world is not actually what the eye is looking at, it's what our brain puts together in the visual cortex (optically, it should be upside down for example, but the brain corrects this). Try moving the focus of your gaze, at an even speed, over a patterned carpet from different angles & you'll see what I mean. If your brain doesn't think it's important, it won't bother resolving it properly. Finding your girlfriend's earring in a place you've already looked at eight times is down to how we perceive.
I found it a constant battle to look at areas totally objectively when looking for particular things, and not allowing myself to see just what a part of my brain expects to see. Having a fixed image in your mind of what you're looking for can impede your ability to see a variation of it. I found that it was more tiring the more you limited your scan to straight down, even though it's theoretically closer.
I reckon somewhere between 30-40 degrees from vertical, with constant "situational" general scans. But it needs to be a methodical, yet natural, scan pattern.
As you say, professional military or civil ASR personnel will be the only ones able to answer this properly. They should be able to advise the best relationship between altitude & airspeed for example.
However, video onto uneven snowed landscape probably favours a somewhat more vertical view.

Another consideration is to look at other aviation sources that perform search and rescue. Civil Air Patrol (CAP) for example, use aircraft routinely to search for aircraft.
A quick read through their Mission Aircrew Reference Text Volume III: Airborne Photographer manual has this to say about the advantages and disadvantages of high and low oblique angles of the target

Low oblique photos only cover a small area, but they do present the photo in
detail and present a more natural view. High oblique photos cover a larger area and
present a detailed view, but only in the foreground (the detail falls off as you get
closer to the horizon).

That being said, they typically are taking pictures of a known disaster or crash site. They do mention though a modification during Hurricane Sandy. Since they didn't know the extent of damage they changed the tactic

The most extreme example is Hurricane Sandy. ...The Maine Wing’s Incident Commander (Col Dan Leclair) stated that this
was a “cultural change” in how CAP takes pictures, as they had to photograph
everything, regardless of whether there was damage or not. This allowed FEMA
to readily pinpoint the areas of greatest need. The aircrews rose to the occasion by flying grid patterns similar to those CAP follows in conducting SAR missions and taking overlapping photos along each leg of the patterns.

Another development they had was about 7 years ago with the introduction of the ARCHER system. I don't know if this is available for purchase from the company that makes the software though.
Basically you tell the ARCHER system what material you're looking for and it tries to find and tag that material as you fly around This wired article explains

called the ARCHER – an acronym for Airborne Real-time Cueing Hyperspectral Enhanced Reconnaissance. The system is capable of panchromatic aerial imaging ... The hyperspectral sensor was built by NovaSol, a Hawaii-based company. 11
A special camera mounted to the belly of the plane transmits detailed images in real time to a flat-screen monitor for operators on the plane to examine... That data is tagged with global-positioning coordinates as well, so searchers can return to an area if ground technicians spot something.

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What does the pressure distribution over a glider's wing look like?
Can someone explain the design of glider wing airfoils and the subsequent pressure distribution over them?
I hypothesize that:

The pressure distribution should form a resultant force in the forward, upward direction, since two components of force are required to maintain equilibrium in the flight of a glider: a) lift - in equal opposition to weight, and b) a forward component of the total reaction generated by the airfoil - in opposition to drag.
Thus I conclude that the angle of attack of a glider airfoil relative to flight direction would be negative, in order to achieve a forward component opposing drag; although it would be positive relative to airflow, such as to generate low pressure over the wings. Either that or the camber is designed such that the two mentioned criterias here, (a) and (b), are upheld.

Should there not be any opposition to drag, I would expect to see the glider decelerate in the forward direction, until the airflow stalls over the wings, and the glider loses lift and falls.
I am also curious to know about the various designs of glider airfoils: a) what are the criterias defining them, and b) how do they differ from those of powered aircraft, if there are any differences at all?

The foward component opposing drag is the glider's weight, not forward lift component (which I don't think is technically possible anyway). Take a close look at forces in flight for a gliding/descending aircraft: Lift equals  only part of total weight. Drag opposes the remaining one

Difference between lift and drag is rather arbitrary, both is caused by more-or-less the same physical phenomena and you can see it as only one (total) aerodynamic force caused by air flowing around the airplane. For practical purposes, this force is usually divided into two components, one, lift, is by definition perpendicular to the direction of motion, second (drag) is parallel. But for the glider, I believe, it is the most easy to think only one total aerodynamic force which is during steady flight always same magnitude and opposite direction to the gravity.

Yes, & that would be induced drag. There is however profile drag & parasitic drag which will always apply on any body moving through a fluid, namely, air in this case. This drag force will have to be opposed in order to maintain an equilibrium descent along the glide angle; how can this be done is one part of my question above.

It is also worth noting that many glider wings (especially the case for trainers) do not have a uniform shape across the whole wing. Many glider wings have different  cross sections to allow for one part of the wing to stall before the rest of the wing. This acts as an alert to the pilot that a stall is imminent but still allowing the pilot to have control using the rest of the wing.

related: https://aviation.stackexchange.com/q/56352/34686

The pressure distribution over a glider's wing is no different from that of a well-designed aircraft. A glider can maintain flight by flying slightly downwards, so the direction of motion is not straight but slightly down. Doing this in rising air will result in sustained flight.

Look at the sketch above: The flight path angle $\gamma$ is negative and in still air the flight path vector $\text{x}_k$ is parallel to the direction of the airspeed. Due to the inclination of the flight path, the lift vector is tilted forward such that it's horizontal component is exactly equal to the horizontal component of drag, and the vector sum of lift, drag, and weight is zero. This is shown on the right where I moved the vectors into a closed sequence which demonstrates that all forces balance.
From the viewpoint of the glider, the lift is pointing straight up and drag directly backwards, but the weight force is slightly tilted forward. In a way, it seems that the thrust of a glider is its weight force.
Now let the whole air packet in which the glider flies move up. The glider will still sink down within this air but relative to the ground it will gain altitude if the upward air velocity is high enough.

Great answer as always, but that picture might cause some miunderstandings; Pet peeve, but too many people confuse this: even though flight path angle is negative,the angle ot attack is still positive.

@Radu094: Yes, you're right. But to show the AoA I would need to add the airplane coordinate system and complicate things even more, so I decided to leave it off.

@quietflyer I gave the drag vector 5° more tilt - now its angle looks exaggerated, but brings the point across. The colours might make things confusing, so I changed those, too.

As drawn, the lift vector is perpendicular to the line of flight and (enlarged a bit) the weight vector is the hypotenuse of the closed triangle.  Though I was once loathe to decompose vectors into vertical and horizontal components, the issue resolves by simply drawing lift, drag and weight vectors as aircraft (not earth) referenced.  One can then clearly see, as in paragraph 3, that sin glide $\theta$ × mg is the propulsive force of the glider (opposing drag).

@PeterKämpf -- I would not say the angle of drag vector looks exaggerated now-- it looks close to parallel with the "Xa =Xk" vector, as it should.  In the vector triangle, the angle between lift and drag does not look less than 90 degrees-- if anything it still looks very slightly greater than 90 degrees (but could be illusion due to shape of arrowhead).  Anyway, the changes helped much, thanks. I'm amazed never found this answer before as it would have been good to link to during when several questions about "what is the thrust/power in a glider", etc came up over the last few years.

I think you are approaching this problem wrong. If you think about how a glider flies, there is no forward force  component on the airfoil that opposes drag. By definition, drag is the force that opposes the motion of the craft and lift is perpendicular to the drag force.
Suppose we drop the glider from some height in a typical flight position (ie fuselage parallel to the ground). The only way this glider is going to fly forward (assuming no tailwind) is by dropping the nose, creating a negative angle of attack with respect to the ground. The lift generated by airfoil here will have both horizontal and vertical components relative to the ground. However, you are also losing altitude in this orientation. The point here is, without initial speed (forward momentum) the only way to create forward speed is to convert potential energy into kinetic energy by losing altitude, so a negative angle of attack will create forward speed, but will cost you altitude.
As to your other questions in regard to glider airfoil design vs propelled aircraft airfoiled design, glider airfoil are typically low camber, low lift, and low drag. This is because while higher camber airfoil generate more lift, they also generate significantly higher drag. This would cause the glider to lose forward momentum faster. Since lift is a function of velocity, your glider loses lift during this time and will fall out of the sky. With propulsion, these airfoil can be used to carry heavier loads (since more lift is generated and the propulsion directly counteracts drag forces). Gliders are typically light and therefore require less lift to stay aloft. The low drag profiles allow them to maintain forward momentum and speed without losing much altitude.
In general, you always want the pressure below the wing to be higher than the pressure above the wing. As soon as this condition is not met, the plane/glider will begin to fall from the sky. Also note that just because the pressure below is higher than the pressure above does not guarantee the aircraft will stay aloft. The upward force generated by this pressure (F=P x Area) must exceed the downward force caused by gravity (F=Mass x Gravity).

This question focuses on a very interesting mechanistic question about what exactly happens when a (model) aircraft is simply dropped from a height.

How does the aircraft start to glide?

If the aircraft is held horizontally, and let go, gravity will cause acceleration. Aerodynamic drag forces now act on the surfaces of the aircraft.  Longitudinal stability will start to point the aircraft in the direction of motion because the center of drag and the center of gravity are off-set, causing rotation, but the wings are still stalled.  Not yet a glider.
In drawing diagrams, at the start, when released, only gravity and drag act on the plane.  The "wind" is striking the underside of the aircraft.  If you are a parachute, those are the vectors drawn.
But even parachutes can move sideways by off-setting center of drag and center of gravity.  The "sideways" motion, combined with downward motion, is the new flight path.
Mechanisticly, what is drawn is a rotation as well as drag and gravity forces.  The "tilt" caused by rotation gives vertical drag a horizontal component, enabling one to change where the parachute will land.

when rotation from drag forces lowers AoA enough to un-stall the aircraft, the wing becomes more efficient at producing upwards and "sideways" forces than vertical drag

The trim of the plane will have rotating forces in balance at an airspeed and AoA sufficient to produce adequate lift.

The glider now flies away in a straight path with the lift, drag, and weight force vectors summing to zero at a constant (steady state) velocity.

Fairly complicated procedure to throw a paper airplane, but illustrating why there is more than just trim to a tail.

Last full paragraph (starts with "Note") of this answer is related, also touches on non-steady-state effects. https://aviation.stackexchange.com/a/56371/34686 Honestly I'd suggest that the present question is best answered by focusing on the steady-state situation, and that the non-steady state situation be addressed by a separate, specific question, if anyone desires to do so. Just my two cents...  (We may already have such questions, not sure... )

Well, 1. mentions "a resultant force in the forward, upward direction".  This happens even before the wing is unstalled.  The unstalled wing just does it so much better.

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text:
What are the rules around the clarity of safety videos? For example, how visible must screens be to each passenger, and how loudly must the video be?
What are the rules around the clarity of safety videos, and who sets them? For example, considering airplanes without individual screens for passengers (which was common in the 90s and 00s): How far away can the screens be? What is the minimum size to ensure visibility? What volume should be set to guarantee every passenger can hear it?


text:
When riding in the flight deck of a large airplane, why does it appear that the airplane moves slowly?
I recently rode in the jumpseat of an A330, and on approach, the visual impression of speed is very different from that of a small airplane or flight simulator.
Specifically, it appears that the airplane moves quite slowly and that the view to the outside is “zoomed in,” whereas in a small airplane or flight simulator, it appears “zoomed out.”
What causes this?

As Doug Henning used to say, "it's an illusion"...

Could it be the position - ie. jumpseat is back away from windows (narrower field of view), whereas flight sim it's pilot eye? or small plane you are right at the front of the plane?  And of course the audio possibly makes a difference too..

This question is about optical illusions, not aviation. as such it probably should be in the Biology stack exchange forum. Voting to close.

I actually have the opposite feeling, that on the jumpseat it actually goes faster than in the sim, talking for the 737/320 now, maybe A330, because sitting further AFT it can create that illusion I guess...